As is well known, the electrolyte used in fuel cells is lost slowly over a period of time, mostly by evaporation into reactant gases passing through the cell. A matrix layer disposed between the electrodes of the cell holds the electrolyte in position between the electrodes during cell operation. The electrolyte within the matrix provides a liquid barrier preventing commingling of the fuel and oxidant gases which flow through the cell on opposite sides of the matrix. If too much electrolyte evaporates from the cell, the reactants may mix with each other through localized areas of the matrix. The electrochemical reaction also stops in those areas.
There is a need to be able to replenish the lost electrolyte at predetermined intervals, the intervals being chosen to assure that the electrolyte volume does not fall below some minimum, safe level. It is desired to be able to refill the cells to a predetermined level and concentration which is uniform from cell to cell. It is too costly to take the stack apart to add acid to the individual cells of the stack. A system is needed to enable the refilling of the cells in a stack without disassembling the stack.
Several U.S. patents representative of the prior art relating to filling fuel cells stacks with electrolyte are U.S. Pat. Nos. 4,366,211; 4,383,008; and 4,383,009. All of these prior art systems require numerous additional stack pieces, as well as the addition of slots, drilled holes, and other modifications to most, if not all, of the fuel cell electrodes, matrices, and separator plates. The extra expense of these features and the difficulty in maintaining proper tolerances for hole alignment from cell to cell are undesirable. Also, in some of these prior art patents a head of electrolyte is established at the bottom end of the stack due to columns of the electrolyte interconnected vertically through the stack. This can result in an uneven distribution of electrolyte within the cells. The stack must also be sealed to hold that acid pressure during filling. None of these patents address the problem of obtaining the desired amount and concentration of electrolyte in each cell.
It is desirable to have an inexpensive system for adding electrolyte to fuel cells. Preferably the system should fill the stack uniformly to a desired electrolyte volume and concentration, and should not require significant redesign or reoperation of fuel cell stack components.